1. Field of the Invention
The present invention is directed to a gas tube type surge protective device using two interfitting electrodes.
2. Description of the Prior Art
A gas tube surge protective device containing two electrodes secured to a cylindrical insulator to provide a vacuum tight enclosure has been described in U.S. Pat. No. 3,702,952. The disclosure of that patent is incorporated herein by reference. The overvoltage arrester disclosed in that patent consists of a first electrode which forms a cylindrical gap with a ceramic housing, together with a circular, wafer-shaped gap and a cylindrical gap in a low deposition space. A starting electrode is produced by sputtering electrode material onto the inside wall of the ceramic housing. The sputtered metal extends into the condensation gap. This type of starting aid requires high static initial ignition values after extended dark storage. Moreover, the effect of the starting aid is deteriorated with current passage, particularly during a surge current load since the metal of the electrodes thereby vaporizes and the expulsion of gases contained therein may raise the ignition voltage.
The gaps used in the low deposition rear spaces of the prior art must be extremely narrow. They should comprise a gap width of about 0.025 mm. This short spacing is required with the design of the prior art in order to prevent a short circuit as a consequence of metal precipitation along the surface of the ceramic housing despite the extremely short gap lengths. In the example given, the gap may have a length of about 1.1 mm overall. Said extremely small gap widths require very precise structuring of the individual parts and an extremely precise assembly. In practice, the ceramic material must also be provided with an inside burnishing which is involved and expensive.